Some chemical carcinogens which can be activated to metabolites mutagenic to Salmonella typhimurium by 9000 Xg supernatant of liver homogenates from different mammalian species are carcinogenic only in certain species. Some other chemical carcinogens are not mutagenic in S. typhimurium with or without in vitro S-9 activation systems. Studies using S. typhimurium tester strains. TA98 and/or TA1535 are conducted to determine whether those false-negative compounds in bacteria mutagenesis test systems can be activated to mutagenic metabolites by hepatocytes from these species. Bacterial cells are treated with carcinogens and hepatocytes in suspension for 2 to 24 hrs. After treatment bacteria are washed and plated both in minimal media and minimal medium supplemented with histidine. Revertants and survival colonies are scored after 2 days incubation at 37 degrees centigrade.